IPv6 is well designed. The model of scopes and zones along with the zone isolation principle is based on solid mathematical standards and can provide straight answers to tricky questions regarding packets with mixed source and destination address scopes. Can a packet with a link-local or ULA address reach the global destination? There is no doubt about that, at least not in IPv6 theory.

Ivan Pepelnjak was discussing the usage of ULA (Unique Local Addresses) recently in one of his blog post at ipSpace. He says: “If the destination IPv6 address is a global IPv6 address and the source host has an ULA address but no global IPv6 address, it tries to use the ULA source IPv6 address (and might reach the destination or not).”. To understand why this can actually work, it is necessary to have some insight about scopes and zone in IPv6, and the basic rules that dictate the packet forwarding within the scope zone. Read the rest of this entry »